The present invention relates to apparatus for actuator positioning and more particularly to an electronically controlled positioner for pneumatic actuators as used in heating, ventilating and air conditioning (HVAC) applications.
Equipment used to control the atmospheric environment within buildings frequently includes sources of heated and chilled water together with air handling units which utilize these sources to affect the temperature of air being introduced into the conditioned space within a building. Such air handling units employ positionable dampers and valves, the former for controlling the flow of air into and out of the building and for air mixing and the latter for controlling the position of valves used to regulate the flow of heated or chilled water. Actuators are frequently used for valve and damper positioning and may be of the mechanically, electrically or pneumatically driven type. However, it is convenient to employ pneumatic actuators for the purpose as such are characterized by relatively low cost and high reliability.
One approach to the control of pneumatic actuators is to provide a pair of electrically-actuated solenoid valves, one of which is coupled to a pneumatic bus installed within a building and the other of which is arranged for selective expulsion of the pneumatic fluid from the actuator. For buildings having no central pneumatic bus, a small, electrically powered air compressor may be provided for individual or small groups of coincidently-controlled actuators. Systems of either the solenoid valve type or of the locally installed air compressor type may be coupled to a programmed computer for controlled actuator positioning and an example of such a system is shown and described in U.S. Pat. No. 4,261,509.
While such arrangements for actuator positioning have heretofore been satisfactory, they tend to be characterized by certain disadvantages. In particular, such systems frequently utilize relatively large and expensive electromagnetic switches. Such switches require undesirably high power levels and are often the source of electrical switching spikes or transients which may adversely affect computer circuitry. Another disadvantage is that such systems are configured only for the separate, relatively expensive mounting of several discrete components which are relatively difficult to adapt to pre-existing actuators used on an HVAC system. Additionally, such apparatus makes no provision for the isolation of switching transients for computerized controller protection nor does it make provisions for the generation of feedback signals indicative of actuator position. Further, it is frequently desirable to employ an electronically controlled positioner which may be adapted for use with low voltage, low power command signals. Such signals may be generated by a computerized controller programmed for the solution of proportional, proportional integral, and/or proportional integral derivative algorithms to provide incremental or proportional control signals. The use of incremental control permits an actuator to be commanded to stroke through a computed percentage of its total available travel span while proportional control permits an actuator to be commanded to stroke to a prescribed position within its span. An electronically controlled positioner for pneumatic actuators which overcomes the aforementioned disadvantages and which incorporates the aforementioned features would be a significant advance over the prior art.